Field of the Invention
This invention relates in general to a method and/or apparatus for measuring atmospheric wind, and in particular to a method and/or apparatus for determining passive Doppler wind measurements using a satellite-based instrument.
Description of the Related Art
Atmospheric wind observations with optical remote sensing techniques that measure Doppler shift have a long heritage. To date, space-based optical measurements of winds in the Earth's atmosphere have been performed using either Fabry-Perot interferometers or Michelson interferometers. In 2018, NASA is expected to launch the Ionospheric Connection Explorer (“ICON”), a satellite in low-Earth orbit, which includes an airglow-measuring instrument known as the Michelson Interferometer for Global High-resolution Thermospheric Imaging (“MIGHTI”). MIGHTI was designed and built by the U.S. Naval Research Laboratory, and it uses the Doppler Asymmetric Spatial Heterodyne (“DASH”) technique discussed in U.S. Pat. No. 7,773,229, incorporated herein by reference, to measure winds. Conventional space-based optical measurements of winds employing Fabry-Perot interferometers or Michelson interferometers use a limb-viewing geometry to detect the Doppler shift of discrete atmospheric emission lines caused by the bulk velocity along the line of sight at the tangent layer. The horizontal wind vector is determined by combining two measurements of the same air mass with orthogonal look directions, typically taken several minutes apart, 45° and 135° from the ram direction of the satellite.
Fabry-Perot Heritage
The High Resolution Doppler Imager (“HRDI”) is an instrument on board NASA's Upper Atmospheric Research Satellite (“UARS”), which was launched on 12 Sep. 1991 as a part of NASA's effort to study the Earth's stratosphere and mesosphere. The TIMED Doppler Interferometer (“TIDI”) is an instrument on board NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (“TIMED”) satellite, which was launched on 7 Dec. 2001 to study the Earth's Mesosphere and Lower Thermosphere. HRDI and TIDI utilize a triple and a single Fabry-Perot interferometer, respectively, to measure emissions having wavelengths (λ) between 550-900 nm.
The Fabry-Perot instruments utilize one or multiple etalons in series to isolate and spectrally resolve the emission line(s) of interest. The spectrum over a narrow wavelength range is obtained directly by imaging the ring pattern produced by the interferometer on a position sensitive detector. Once the spectrum is obtained, the wind speed can be derived from the line position. The temperature can be determined from either the line width or a line ratio. The biggest technical challenge for the Fabry-Perot instruments lies in achieving the required etalon alignment tolerances (better than ˜λ/20) and maintaining this alignment during flight. Although many spectral resolution elements are measured in parallel, the solid angle Ω for a single resolution element is determined by the resolving power R (i.e. Ω=2π/R) which can be small at the resolving power required for Doppler measurements. Since the high resolving power necessitates a small solid angle, a large interferometer aperture may be required to obtain adequate signal on faint emissions. This results in a larger, heavier instrument.
Stepped Michelson Heritage
The Wind Imaging Interferometer (“WINDII”) was launched on NASA's UARS on 12 Sep. 1991 and operated until 2003. WINDII used an all-glass, field-widened, chromatically, and thermally compensated, phase-stepped Michelson interferometer (also termed Stepped Fourier Transform Spectrometer or stepped FTS). Several other versions of phase-stepped interferometers have been built or proposed for the measurement of telluric winds and winds on Mars.
The basic principle behind all phase-stepped Michelson interferometers is to measure a minimum of three, but typically four, interferogram points of a single isolated atmospheric emission line. The phase points are spaced by ˜λ/4 (90°) about a step (or offset) in optical path difference (“OPD”) that is large enough to be sufficiently sensitive to both wind speed, which results in a phase shift at high OPD, and temperature, which results in a variation in modulation depth.
This basic principle is illustrated with reference by way of illustration to FIG. 1, which shows a stylized, schematic interferogram as it would be recorded by a conventional scanning Michelson interferometer viewing an isolated, single, temperature-broadened, Gaussian emission line. The carrier frequency of the fringe pattern is determine by the central wavenumber of the emission, which is Doppler shifted by the wind speed. For a predominantly temperature broadened line, the width of the interferogram envelope is a measure of the temperature, with a higher temperature corresponding to a narrower envelope. Zero path difference is at the center of the plot, where the visibility of the fringes is maximal, and the maximum path difference is at the edges. The thin curve in the plot shows the intensity vs. optical path difference for a Gaussian emission line as it would be recorded by a scanning Michelson interferometer scanned over the path difference shown. The thick curve in the plot illustrates the residual obtained by taking the difference between two interferograms each corresponding to a different wind speed, which causes them to have slightly different carrier frequencies. The maximum response of the measurement to wind speed is at path difference POPT where the amplitude of the difference signal is maximal. Assuming a temperature broadened, Gaussian line profile with width σD:
                              σ          D                =                              σ            0                    ⁢                                    kT                              mc                2                                                                        (        1        )            the optimum path difference is:
                              P          OPT                =                  1                      2            ⁢                                                  ⁢                          πσ              D                                                          (        2        )            where σ0 is the wavenumber of the line center, k is Boltzmann's constant, m is the molecular or atomic mass of the emission source, T is the temperature, and c is the speed of light. One of ordinary skill in the art will readily appreciate that the fringe frequency in FIG. 1 has been greatly reduced for ease of illustration and understanding. A real interferogram taken with a Michelson interferometer for an emission line in the visible spectral range would produce on the order of 105 fringes between path differences 0 and POPT under typical atmospheric conditions.
Determining Doppler shifts with a phase-stepped Michelson interferometer requires the isolation of a single emission line with a pre-filter. A fit of the interferogram phase at the four measured samples is then possible, which can subsequently be used to determine the Doppler frequency shift. If the line is spectrally close to other emissions in the spectrum, the pre-filter has to be extremely narrow, which can be achieved by an additional, standard, Fabry-Perot etalon prefilter, with all of its attendant difficulties and the resulting reduction in throughput.
Several stepped FTS techniques have been used to measure the four phase points. The WINDII instrument uses piezoelectric actuators to move one mirror of the interferometer. The Mesospheric Imaging Michelson Interferometer. (“MIMI)” instrument uses a segmented mirror with four sections at different OPD, which avoids moving the mirror. The Waves Michelson Interferometer (“WAMI”) version, designed for the Earth's atmosphere, proposes a moving, segmented mirror, allowing the simultaneous measurement of two emission lines with a two-step mirror scan. A phase-stepped Michelson interferometer has also been proposed for Mars using a non-segmented, mirror moved by piezo actuators.